The present invention relates generally to the field of communications. More particularly, the invention relates to a frequency hopping spread spectrum technique for accepting and sorting complex waveforms to lock onto a desired remote signal.
Fixed network communication systems may operate using wire line or radio technology. Wire line technologies include utilizing the utility distribution lines and/or telephone lines. Wireless technologies may utilize the 902-928 MHz range, which can operate without a FCC license through the use of frequency hopping spread spectrum (FHSS) transmission, which spreads the transmitted energy over the band. According to FCC Regulations, for frequency hopping systems operating in the 902-928 MHz band, total output is as follows: 1 watt for systems employing at least 50 hopping channels; and, 0.25 watts for systems employing less than 50 hopping channels, but at least 25 hopping channels. See, 47 U.S.C. xc2xa7 15.247.
FHSS systems meet the FCC specification by communicating to remote communication devices in synchronization, both in time and frequency. Using this approach, all devices know when to hop to the next channel in the sequence and what the next sequence channel is. A known FHSS system utilizes a hop rate that is faster than the data rate to send multiple sets of randomly selected frequencies in each message to distribute the transmitted energy over the communication band. This distribution is one of the FCC requirements to operate in the ISM band.
A disadvantage of the above is that it requires all devices to include a real time clock, which adds to the cost of the device. In addition, some type of battery storage system is required to maintain the real time clock in the event power should be removed from the device. Further, the requirement to step rapidly through the frequencies constrains the design of such devices and further limits cost reduction.
There have been attempts to utilize remote devices that operate asynchronously that gain synchronization with the transmitter by using a scanning receiver. One example of such a receiver is that of Harmon, U.S. Pat. No. 4,328,581. However, spurs and other unwanted signals interfere with the synchronization process in such receivers. Another problem with such systems is that the receiver is scanning channels based on a list and may be scanning channels other than the channel on which a preamble is being sent. To compensate for this, the transmitter must send the preamble for a period of time long enough to allow the receiver to scan for it and develop a bit timing. In these systems, spurs may be decoded as valid preambles, which adversely affects performance. Further, crosstalk problems may arise if weak signals from the transmitter are captured in the receiver scan sequence prior to determining the desired transmit channel.
Therefore, there is a need for a FHSS communication device that is cost efficient, meets FCC requirements for power distribution in the ISM band, and includes provisions for preventing unwanted signals from capturing the scanning receiver. The present invention is directed to these, as well as other, needs in the art.
The present invention addresses the needs identified above in that it provides for a novel method and apparatus that utilizes frequency hopping spread spectrum communications. In accordance with the present invention, there is provided an asynchronous frequency hopping spread spectrum receiver that includes an integrated circuit transceiver providing a PLL lock signal and a received signal strength indicator signal, and a microcontroller that receives the PLL lock signal and the received signal strength indicator signal. The receiver scans a predetermined list of channels and the microcontroller converts the received signal strength indicator signal for each scanned channel to a digital value representative of the received signal strength indicator signal and stores each the digital value in a received signal strength indicator list. The microcontroller next determines a subset of channels to scan for a preamble pattern. The subset of channels may be determined from a predetermined number of highest digital values stored in the received signal strength indicator list.
In accordance with a feature of the invention, the microcontroller rescans the predetermined list of channels a predetermined number of times and determines the subset of channels from the highest digital values after completing the predetermined number of rescans. A channel corresponding to a highest digital value contained in the subset of channels is scanned for the preamble pattern. If the channel corresponding to the highest digital value does not contain the preamble pattern, a next channel corresponding to a next highest value in the subset of channels is scanned for the preamble pattern.
In accordance with another aspect of the invention, there is provided a method of scanning for a preamble bit pattern in an asynchronous frequency hopping spread spectrum receiver. The method includes scanning a predetermined list of channels; converting a received signal strength indicator signal for each scanned channel to a digital value representative of the received signal strength indicator signal; storing the digital value in a received signal strength indicator list; and determining a subset of channels to scan for a preamble pattern, the subset of channels being determined from channels corresponding to a predetermined number of highest digital values stored in the received signal strength indicator list.
In accordance with yet another aspect of the invention, there is provided a process for scanning for a preamble bit pattern in an asynchronous frequency hopping spread spectrum receiver. The process includes scanning a predetermined list of channels a plurality of times to determine a subset of channels having higher signal strengths relative to all channels in the predetermined list, and successively scanning each channel within the subset of channels for a preamble bit pattern beginning with a channel having the highest signal strength until the preamble pattern is found.
These and other aspects of the present invention will be described in the following detailed description of the invention.